Such an input stage finds application in particular in a high linearity hi-fi amplifier.
In such an amplifier, it is common to use at the input, a digital-to-analog converter such as the component PCM 1792 from Texas Instruments. This digital-to-analog converter has a current output, so that the analog signal is modulated in intensity.
Insofar that the amplification stage placed downstream uses at the input a modulated voltage, a current-voltage converter should be placed between the digital-to-analog converter and the amplification stage strictly speaking.
The digital-to-analog converters with a current output are particularly appreciated for their very low harmonic distortion rate which they may attain. The difficulty consists of utilizing the properties of such digital-to-analog converters via a current-voltage conversion stage which itself has no impact on the performances of the digital-to-analog converter.
The current sources contained in such digital-to-analog converter are connected to one or several outputs connected to a virtual ground or further connected between a ground and one or several outputs connected to a virtual ground. Such connections are traditionally achieved by means of an operational amplifier circuit.
A virtual ground is a fixed potential.
In this way, all the transistors of the digital-to-analog converter operate with constant current and voltage, therefore optimally, regardless of the modulation of the output signal.
In order to preserve this lack of distortion, the current-voltage converters placed downstream may also, in an implementation different from that of an operational amplifier circuit, comprise specific stages said to be with a «common gate» based on MOSFET transistors (Metal Oxide Semiconductor Field Effect Transistors) or said to with a «common base» based on bipolar transistors. Such stages with a common gate or base operate in an open loop.
This type of circuit comprising transistors with a common base or gate, more commonly called “cascode” transistors, is described in document WO 2011/107671. In this document, the transistors of the stages with a common base or gate are connected at the level of their source to current sources.
At least one of the current sources is able to impose a biased current. The other current sources are notably able to cancel out the continuous component from the digital-to-analog converter so that only the variable component from the digital-to-analog converter is converted into a voltage by the current-voltage converter.
The cascode stage gives the possibility of guaranteeing that the current sources do not exhibit voltage variations on their terminals when the voltage at the voltage output of the current-voltage converter varies, and this even when this voltage varies by several tens of volts.
Further, such a cascode stage imposes a constant potential at the current output of the digital-to-analog converter, regardless of the voltage at the voltage output of the current-voltage converter.
Therefore, the quality of the information contained in the current from the digital-to-analog converter is preserved when this current is converted into a voltage by the resistance.
However, these current sources are notably, at the origin of substantial current consumption and at the origin of heating of the input stage.